1. Field of the Invention
The present invention relates to an optical film, and a polarizing plate and a liquid crystal display device using the same.
2. Description of the Related Art
A liquid crystal display device (LCD) has such large advantages as thin type, lightweight and low power consumption as compared with a CRT (Cathode Ray Tube). The liquid crystal display device is composed of a liquid crystal cell and a pair of polarizing plates arranged on both sides of the liquid crystal cell. The liquid crystal cell is composed of a rod-shaped liquid crystalline compound (liquid crystalline molecule), 2 substrates for sealing the same and electrode layers for applying voltage to the rod-shaped liquid crystalline compound. In order to align the sealed rod-shaped liquid crystalline compound, the 2 substrates are provided with an alignment film. In order to remove color of images displayed on the liquid crystal cell, an optical film (retardation plate) is often arranged between the liquid crystal cell and the polarizing plate. A laminated body of the polarizing plate and the optical film functions as an elliptically polarizing plate. Such function as widening a view angle of the liquid crystal cell is sometimes given to the optical film. A stretched birefringent film has been conventionally used as the optical film.
In place of a stretched birefringent film, use of an optical film having an optically anisotropic layer including a discotic liquid crystalline compound is also proposed (for example, refer to JP-A-6-214116, U.S. Pat. No. 5,583,679, U.S. Pat. No. 5,646,703 and DE-A-3911620). The optically anisotropic layer is formed by aligning a discotic liquid crystalline compound and fixing the aligned stated. A discotic liquid crystalline compound generally has a large birefringent index. In addition, a discotic liquid crystalline compound has multimodal alignment shapes. Accordingly, by using a discotic liquid crystalline compound, it is possible to manufacture an optical film having optical properties that can not be obtained from conventional stretched birefringent films.
There is proposed a liquid crystal display device using a liquid crystal cell of a bend alignment mode in which rod-shaped liquid crystalline compounds are aligned in substantially opposite directions (symmetrically) in the upper portion and the lower portion of the liquid crystal cell (for example, refer to U.S. Pat. No. 4,583,825 and U.S. Pat. No. 5,410,422). Since rod-shaped liquid crystalline compounds are aligned symmetrically in the upper portion and the lower portion of the liquid crystal cell, the liquid crystal cell of the bend alignment mode has a self-optical compensatory function. Consequently, this liquid crystal mode is also called an OCB (Optically Compensatory Bend) liquid crystal mode. A liquid crystal display device of the bend alignment mode has such advantage as quick response speed.
The bend alignment mode has such characteristics as a wide view angle and quick response speed as compared with general liquid crystal modes (TN mode, STN mode). However, when compared with a CRT, a further improvement is necessary. In order to further improve a liquid crystal display device of a bend alignment mode, use of an optical film is considered as is the case for general liquid crystal modes. However, conventional optical films composed of a stretched birefringent film were insufficient in an optical compensatory function for a liquid crystal display device of the bend alignment mode. As described above, in place of a stretched birefringent film, use of an optical film having an optically anisotropic layer including a discotic liquid crystalline compound and a support is proposed. Further, liquid crystal display devices of the bend alignment mode using an optical film including a discotic liquid crystalline compound are also proposed (for example, refer to JP-A-9-197397 and WO 96/37804). By using an optical film including a discotic liquid crystalline compound, the view angle of a liquid crystal display device of the bend alignment mode is significantly improved.
Such problem is indicated that, when an optical film including a discotic liquid crystalline compound is used for a liquid crystal display device of the bend alignment mode, light with a specific wavelength leaks to color the displayed image (for example, refer to JP-A-11-316378). There is described that this coloring is caused by wavelength dependence of the transmittance of an elliptically polarizing plate (laminated body of a polarizing film and an optical film). Since wavelength dependence of the anisotropy of a liquid crystalline compound used for a liquid crystal cell differs from wavelength dependence of the anisotropy of an optical film (for example, discotic liquid crystal), light with a short wavelength (blue) generally leaks in a liquid crystal display device of the bend alignment mode. However, by matching the chromatic dispersion of a liquid crystalline compound with that of an optical film (discotic liquid crystal), hue at the time of black level is improved.
And it is reported that, by arranging an optically anisotropic layer and a polarizing film so that an angle between an average direction of an orthogonal projection of the normal line of the disc plane of the discotic liquid crystalline compound onto the optically anisotropic layer and the in-plane transmission axis of the polarizing film substantially is 45°, the largest optical compensatory effect for a liquid crystal cell of the bend alignment mode can be obtained. With regard to a bend alignment liquid crystal device using an optical film including a discotic liquid crystalline compound, various methods have been proposed to lower hue alteration and prevent gradation reversal (for example, refer to Japanese Patent No. 3056997 and JP-A-2002-40429).